Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus

Summary

Here, we present a protocol for the synthesis of two carbonyl-decorated carbenes. The protocol makes these interesting compounds readily available to chemists of all skill levels. In addition to the synthesis of these two carbenes, their use in the activation of white phosphorus is also described.

Abstract

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures in multi-gram scale quantities. The goal of this manuscript is to clearly detail how to handle and prepare these unique carbenes such that a synthetic chemist of any skill level can work with them. The two carbenes described are a diamidocarbene (DAC, carbene 1) and a monoamidoaminocarbene (MAAC 2). These carbenes are highly electron-deficient and as such display reactivity profiles that are atypical of more traditional N-heterocyclic carbenes. Additionally, these two carbenes only differ in their electrophilic character and not their steric parameters, making them ideal for studying how carbene electronics influence reactivity. To demonstrate this phenomenon, we are also describing the activation of white phosphorus (P₄) using these carbenes. Depending on the carbene used, two very different phosphorus-containing compounds can be isolated. When the DAC 1 is used, a tris(phosphaalkenyl)phosphane can be isolated as the exclusive product. Remarkably however, when MAAC 2 is added to P₄ under identical reaction conditions, an unexpected carbene-supported P₈ allotrope of phosphorus is isolated exclusively. Mechanistic studies demonstrate that this carbene-supported P₈allotrope forms via a [2+2] cycloaddition dimerization of a transient diphosphene which has been trapped by treatment with 2,3-dimethyl-1,3-butadiene.

Introduction

Stable carbenes have emerged as ubiquitous reagents in homogeneous catalysis¹, organocatalysis², materials science^3,4, and more recently main group chemistry^5-9. In the context of the latter, stable carbenes have recently been used in the activation and functionalization of white phosphorus (P₄)^5-9. The ability to directly convert P₄ into organophosphorus compounds has become a topical research objective in an effort to develop “greener” methods that circumvent the use of chlorinated or oxychlorinated phosphorus precursors. Despite their widespread use, the preparation and handling of carbenes and reactive compounds such as P₄ can be a daunting task. For this reason, we have written this manuscript to provide a clear and concise protocol that will allow synthetic chemists of all skill levels to synthesize and manipulate two very unique stable carbenes. Additionally, the activation of P₄ using the described carbenes is detailed.

Herein we detail a protocol for the synthesis of two electron-deficient carbonyl decorated carbenes. We have chosen these carbenes because they differ only in their electrophilic properties, and not their steric parameters, making them ideal for studying the effects of carbene electronics on reactivity. The importance of carbene electronics with regard reactivity is exemplified by two similar compounds of the general formula carbene-P₂-carbene that have been reported by Bertrand and Robinson^5,8. Bertrand’s P₂ derivative is supported by two cyclic alkyl amino carbene (CAAC) ligands, and is structurally, photophysically, and electrochemically different than Robinson’s compound which is a P₂ fragment supported by two N-heterocyclic carbenes (NHCs)^5,8. Indeed, Bertrand’s P₂ complex is characterized as a yellow solid that features carbene-to-phosphorus double bonds in the solid state, whereas the derivative reported by Robinson is a dark red solid that contains NHC→P dative bonds. This structural difference also manifests itself electrochemically such that Robinson’s compound contains more electron-rich phosphorus centers that can undergo reversible 1- or 2-electron oxidations in contrast to Bertrand’s compound which can only undergo a single reversible oxidation¹⁰.

Based on the studies described above, we became interested in studying the activation of P₄ using the highly electrophilic diamido- and monoamidoamino carbenes to determine if novel carbene-stabilized allotropes of phosphorus could be prepared. We focused on diamidocarbene (DAC) 1, and monoamidoamino carbene (MAAC) 2 which differ only in their respective electrophilicities to interrogate what role carbene electronics play in P₄ activation. Interestingly when the more electrophilic DAC is used, a tris(phosphaalkenyl)phosphane (3) could be isolated as the exclusive product, whereas when a MAAC is used, a carbene-stabilized P₈ allotrope (4) can be obtained¹¹. We also interrogated the mechanism for the formation (4), and found that it is formed via a [2+2] cylcoaddition dimerization reaction of a transient diphosphene. The existence of this diphosphene was confirmed by trapping it with 2,3-dimethyl-1,3-butadiene to furnish the [4+2] cycloaddition adduct 5. The protocol for synthesizing these carbonyl-decorated carbenes and their corresponding P₄ activated compounds is described herein.

Protocol

1. Synthesis of Diamidocarbene (Compound 1)

1. Connect an oven-dried 100 ml Schlenk flask to a high performance vacuum manifold, evacuate it and flush with nitrogen. Add a stir bar to the flask and cap with a rubber septum. Weigh out N,N’-dimesitylformamidine¹² (1.5 g, 5.35 mmol) and add it to the flask while flushing with nitrogen.
   1. Add (via dry, deoxygenated syringes) 30 ml of dry, degassed dichloromethane (DCM) followed by triethylamine (1.1 ml, 8.0 mmol, 1.5 equiv.). Cool the resulting solution to 0 °C in an ice bath.
   2. Add dropwise (via a dry, deoxygenated syringe) dimethylmalonyl dichloride (0.75 ml, 5.60 mmol, 1.05 equiv.) to the cooled solution. Upon addition, allow the solution to stir at 0 °C for 1 hr under an atmosphere of nitrogen. Once the solution has stirred for 1 hr, remove all the volatile materials under vacuum.
   3. Add a solvent mixture of dry, degassed hexanes:DCM (2:1 by volume, 24 ml total volume) to the residue in the reaction flask. Let the mixture triturate for 10 min.
   4. Meanwhile, attach a filter tube equipped with a medium porosity glass frit to the top of an oven-dried 100 ml Schlenk flask (see Figure 1). To the filter tube, add enough oven-dried Celite to create a filter plug approximately 2 inches tall. Seal the filter tube with a rubber septum, and pull vacuum, on the system by attaching the Schlenk flask to the vacuum manifold.
      
      Figure 1. Filtration apparatus assembly for removal of [HNEt₃][Cl] from carbene precursors 1-HCl and 2-HCl. This apparatus is also used to remove NaCl formed during the synthesis of carbenes 1 and 2.
   5. Filtration apparatus assembly for removal of [HNEt₃][Cl] from carbene precursors 1-HCl and 2-HCl. This apparatus is also used to remove NaCl formed during the synthesis of carbenes 1 and 2.
   6. Once the filtration apparatus has been assembled and is under vacuum, transfer the white suspension in the reaction flask via cannula into the filter tube. Be sure to periodically pull vacuum on the collection Schlenk flask to ensure that all of the solution filters through the Celite.
   7. Wash the Celite by adding a solvent mixture of dry, degassed hexanes:DCM (2:1, volume, 18 ml total volume) using a syringe through the rubber septum. Again, periodically pull vacuum on the collection Schlenk flask to ensure that all of the solution filters through the Celite.
   8. Disconnect the collection Schlenk flask from the filter tube under a flush of nitrogen, then seal the collection Schlenk flask with a glass stopper. Remove all of the solvent from the collection Schlenk flask under vacuum to afford the precursor for diamidocarbene 1 (1-HCl) as an air/moisture sensitive white powder in approximately 92% yield (2.04 g). The product can be verified by ¹H and ¹³C NMR spectroscopy (CDCl₃)^13,14. Transfer compound 1-HCl into a glove box for storage prior to the next step.
2. To prepare the diamidocarbene 1, first transfer an oven-dried 100 ml Schlenk flask equipped with a stir bar and a glass stopper into a nitrogen-filled glove box.
   1. Weigh out carbene precursor 1-HCl (0.600 g, 1.45 mmol) and sodium hexamethyldisilazide (NaHMDS, 0.267 g, 1.46 mmol) and place both solids into the Schlenk flask.
3. Add dry, degassed benzene (25 ml) to the two solids in the Schlenk flask and then stopper the flask. At this point, the Schlenk flask can be removed from the glove box.
4. Stir the solution of the carbene at RT for 30 min. Throughout the reaction, the solution will become a turbid yellow-orange color. During this time, set up a filtering apparatus similar to the one described above (using a 1 inch plug of Celite) for the synthesis of 1-HCl.
5. Filter the carbene solution (to remove precipitated NaCl) as described for the precursor 1-HCl. Once the solution has been filtered, remove all of the volatiles using vacuum to afford the crude carbene 1 as a yellow-orange powder. Further purify carbene 1 by washing the solid with cold hexanes (~10 ml) to afford analytically pure compound as an air/moisture sensitive white powder in approximately 85% yield (0.462 g). Verify the product by ¹H and ¹³C NMR spectroscopy (C₆D₆)¹³.

2. Synthesis of Monoamidocarbene (Compound 2)

1. Connect an oven-dried 250 ml Schlenk flask to a high performance vacuum manifold, evacuate it and flush with nitrogen. Add a stir bar to the flask and cap with a rubber septum. Weigh out N,N’-dimesitylformamidine (3.00 g, 10.70 mmol) and add it to the flask while flushing with nitrogen.
   1. Add (via dry, deoxygenated syringes) 125 ml of dry, degassed DCM followed by triethylamine (2.25 ml, 16.05 mmol, 1.5 equiv.). Cool the resulting solution to 0 °C in an ice bath.
   2. Add dropwise (via a dry, deoxygenated syringe) 3-chloropivaloyl chloride (1.54 ml, 11.77 mmol, 1.1 equiv.) to the cooled solution. Upon addition, allow the solution to stir at 0 °C for 30 min under an atmosphere of nitrogen. Then gradually warm the solution to RT, and then remove all the volatile materials under vacuum. After the solvent is removed, a white solid residue will remain.
   3. Add toluene (200 ml) to the white solid and allow the suspension to triturate for 1 hr. Then filter the mixture over a 1 inch plug of Celite using a medium porosity glass fritted Büchner funnel.
   4. Transfer the toluene solution to a 500 ml round bottom flask equipped with a stir bar. Connect a reflux condenser to the flask and heat the solution to reflux (110 °C) for 16 hr. Over the course of the reaction, a white precipitate will form.
   5. After 16 hr allow the suspension to cool to RT. During this time, more solid will precipitate from the solution. Collect the solid via vacuum filtration and wash the solid with cold toluene (3 x 20 ml).
   6. Dry the obtained white solid using vacuum to afford the precursor for monoamidocarbene 2 (2-HCl) as an air stable white powder in approximately 91% yield (3.32 g). The product can be verified by ¹H and ¹³C NMR spectroscopy (CDCl₃)¹⁵. Transfer compound 2-HCl into a glove box for storage prior to the next step.
2. To prepare the monoamidocarbene 2, first transfer an oven-dried 100 ml Schlenk flask equipped with a stir bar and a glass stopper into a nitrogen-filled glove box.
   1. Weigh out carbene precursor 2-HCl (0.500 g, 1.25 mmol) and NaHMDS (0.241 g, 1.32 mmol) and place both solids into the Schlenk flask.
   2. Add dry, degassed benzene (45 ml) to the two solids in the Schlenk flask and then stopper the flask. At this point, the Schlenk flask can be removed from the glove box.
   3. Stir the solution of the carbene at RT for 30 min. During the reaction, the solution will become a turbid yellow color. Meanwhile, set up a filtering apparatus similar to the one described above for the synthesis of 1-HCl using a 1 inch plug of Celite.
   4. Filter the carbene solution (to remove precipitated NaCl) as described for carbene 1. Once the solution has been filtered, remove all of the volatiles using vacuum to afford the crude carbene 2 as a tan powder. Further purify carbene 2 by washing the solid repeatedly with pentane to afford analytically pure compound as an air/moisture sensitive white powder in approximately 62% yield (0.309 g). Verify the product by ¹H and ¹³C NMR spectroscopy (C₆D₆)¹⁵.

3. Synthesis of a Tris(phosphaalkenyl)phosphane (Compound 3)

Caution Statement: White phosphorus is extremely pyrophoric as well as toxic and should be handled cautiously in a glovebox whenever possible.

1. To make a tris(phosphaalkenyl)phosphane (compound 3), weigh out diamidocarbene 1 (0.100 g, 0.266 mmol, 3 equiv.) and white phosphorus (P₄, 0.011 g, 0.089 mmol, 1 equiv.) inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P₄ is light sensitive.
2. Add the two solids to a 20 ml glass vial that is wrapped in aluminum foil. Add dry, degassed diethyl ether (Et₂O, 10 ml) to the solids and then cap the vial. Stir the slurry in the dark for 2 hr. During the course of the reaction, a bright red-orange precipitate will form.
3. Isolate the red solid via filtration using a 10 ml medium porosity glass fritted Büchner funnel. Wash the red solid with Et₂O (4 x 5 ml) and then dry it under vacuum to afford compound 3 as an analytically pure air stable compound in approximately 82% yield, 0.092 grams (based on P₄). Verify the product by ¹H and ³¹P NMR spectroscopy (C₆D₆)¹¹.

4. Synthesis of a Carbene-stabilized P₈ Allotrope (Compound 4)

1. Method A
   1. To make a carbene-stabilized P₈ allotrope (compound 4), weigh out monoamidocarbene 2 (0.100 g, 0.276 mmol, 3 equiv.) and P₄ (11.4 mg, 0.092 mmol, 1 equiv.) inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P₄ is light sensitive.
   2. Add the two solids to a 20 ml glass vial that is wrapped in aluminum foil. Add dry, degassed diethyl ether (Et₂O, 10 ml) to the solids and then cap the vial. Upon addition of the ether, the fleeting dark green color rapidly changes to bright orange. Stir the slurry in the dark for 2 hr. During the course of the reaction, a bright orange precipitate will form.
   3. Isolate the orange solid via filtration using a 10 ml medium porosity glass fritted Büchner funnel. Wash the orange solid with Et₂O (4 x 2 ml) and then dry it under vacuum to afford compound 4 as and analytically pure air stable compound in approximately 51% yield, 39.5 mg (based on P₄). Verify the product by ¹H and ³¹P NMR spectroscopy (THF-D8)¹¹.
2. Method B
   1. Weigh out monoamidocarbene 2 (0.100 g, 0.276 mmol, 2 equiv.) and P₄ (17.1 mg, 0.138 mmol, 1 equiv.) inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P₄ is light sensitive.
   2. Add the two solids to a 20 ml glass vial that is wrapped in aluminum foil. Add dry, degassed hexanes (10 ml) to the solids and then cap the vial. Upon addition of the ether, the fleeting dark green color rapidly changes to bright orange. Stir the slurry in the dark for 2 hr. During the course of the reaction, a bright orange precipitate will form.
   3. Isolate the orange solid via filtration using a 10 ml medium porosity glass fritted Büchner funnel, and then dry it under vacuum to afford compound 4 as and analytically pure air stable compound in approximately 75% yield, 87.7 mg (based on P₄). Verify the product by ¹H and ³¹P NMR spectroscopy (THF-D8).¹¹

5. Trapping a Transient E-1,2-bis(phosphaalkenyl)diphosphene via [4+2] Cycloaddition: Synthesis of Compound 5

1. To prepare compound 5, weigh out monoamidocarbene 2 (0.300 g, 0.828 mmol, 2 equiv.) and P₄ (51.3 mg, 0.414 mmol, 1 equiv.) separately inside of a nitrogen-filled glove box with the lights turned off. Turn off as many lights in the lab during these first few steps as P₄ is light sensitive.
2. Add the P₄ to a 20 ml glass vial, and then add dry, degassed hexanes (18 ml) to the vial. Next, add 2,3-dimethyl-1,3-butadiene (2 ml) to the P₄ suspension in hexanes.
3. While the hexane/P₄ suspension is rapidly stirring, add the carbene 2 as a solid in one portion. The suspension will instantly become bright yellow. Over a period of about 10 min, all of the solids will dissolve followed by the precipitation of a bright yellow solid. At this point, let the suspension stir for 4 hr.
4. After 4 hr, isolate the yellow solid via filtration using a 10 ml medium porosity glass fritted Büchner funnel. This yellow solid is compound 5 (> 90% purity by both ¹H and ³¹P NMR). Concentrate the yellow supernatant solution to dryness and combine the yellow residue with the filtered yellow solid.
5. To purify compound 5, recrystallize the combined yellow solids from a 1:3 (by volume) DCM:hexanes solution (12 ml total volume) in a -30 °C freezer in the glovebox O/N. This procedure will afford 5 as analytically pure air stable yellow crystals in approximately 71% yield, 0.301 grams (based on P₄). Verify the product by ¹H and ³¹P NMR spectroscopy (C₆D₆)¹¹.

Results

The ability to isolate a tris(phosphaalkenyl)phosphane such as 3 or the P₈-allotrope (4) from white phosphorus relies on the use of an electrophilic carbene to activate the P₄ tetrahedron^11,16. Therefore, it is critical to prepare carbenes with enhanced π-acidity, and by extension electrophilicity. Figure 2 illustrates the synthesis of carbene precursor 1-HCl and its subsequent deprotonation to afford the diamidocarbene <...

Discussion

A straightforward procedure for generating carbonyl-decorated carbenes and their application in the activation of white phosphorus is presented here. The critical steps in the protocol for synthesizing the carbenes are: (a) make sure all solvents are properly dried before use, (b) make sure the addition of acid chlorides to the formamidine is done very slowly, (c) if the Celite is not oven-dried for a minimum of 12 hr at 180 °C, hydrolysis of the 1-HCl as well as carbenes 1 

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2,4,6-Trimethylaniline	Alfa Aesar	AAA13049-0E	98%
Triethylorthoformate	Alfa Aesar	AAA13587	98%
Dimethylmalonyl dichloride	TCI	D2723	>98%
3-Chloro-pivaloyl chloride	Aldrich	225703-25G	98%
Triethylamine	Alfa Aesar	AAA12646	Stored over dried, activated 3 Å molecular sieves
Celite™ 545	EMD	CX0574-3D	Oven-dried at 180 °C for a minimum of 12 hr
Sodium hexamethyldisilazide	Across	200014-462	95+%
2,3-Dimethyl-1,3-butadiene	Alfa Aesar	AAAL04207-09	98%
Dichloromethane	EMD	DX0835-5	Purified through solvent purification system, or standard methods
Tetrahydrofuran	Mallinckrodt	8498-09	Purified through solvent purification system, or standard methods
Hexanes	EMD	HX0299-3	Purified through solvent purification system, or standard methods
Benzene	EMD	BX0220-5	Purified through solvent purification system, or standard methods
Toluene	BDH	1151-19L	Purified through solvent purification system, or standard methods
White phosphorus	Generously donated from the Texas A&M chemistry store room.		Purified through sublimation and transferred directly into a glovebox while under vacuum in the sublimator